Retroreflectors, which are devices for returning a beam of incident energy and preferably incident light in a direction parallel to the incident beam, are well known in the art. Typical retroreflectors are known to utilize corner cubes or right triangle prisms, as illustrated in White U.S. Pat. No. 4,349,598 and in Heasley et al U.S. Pat. No. 3,980,393.
As is known in the art, such devices rely on repeated internal reflection of an incident ray by a number of reflecting surfaces to obtain the appropriate reflected ray. Such an arrangement, as shown in U.S. Pat. No. 3,980,393, may be combined with a forward lens in order to indicate a reflective area larger than the actual area of the cube-corner array used therein.
Other retroreflective devices are known to utilize glass beads adjacent a reflective material to provide reflection in the desired direction. Such devices are typically identified by the terminology "cat's-eye" retroreflectors and are analyzed in "Paraxial Ray Analysis Of A Cat's-eye Retroreflector", J. J. Snyder, Applied Optics, Volume 14 No. 8, page 1825 (1975). Glass spheres or beads may be used to replace the cat's-eye lens, as described in Eigenmann U.S. Pat. Nos. 3,964,820 and 4,072,403, and in Goldman U.S. Pat. No. 3,405,025, respectively.
However, both the cube-corner type reflector and the cat's-eye type reflector suffer disadvantages. For example, it is difficult to construct small size cube-corners and to assemble the same into an array. Moreover, internal corners of the cubes so assembled provide interference fringes in the reflected ray. Yet another difficulty with cube-corner reflectors is the requirement for three optically flat and reflectively coated surfaces intersecting at precisely orthogonal angles. Production of small sized and high quality orthogonal intersections of three such optically flat surfaces, required to form a pseudoconjugator array of the type described in "Wavefront Compensation with Pseudoconjugation," T. R. O'Meara, Optical Engineering 21, 271 (1982), is particularly difficult.
Such difficulties lead to inaccuracies of results provided in precise applications such as surveying and metrology, image processing, optical communications and other laser systems.
Additionally, in cat's-eye type reflectors utilizing glass beads, it is difficult to return large fractions of the incident light in a very small angle about the incident direction. Further, such retroreflectors require two precision finished lens surfaces and a precision finished mirror surface for the standard lens-mirror configuration. Additionally, a fixture is required to align and hold the lens in position in front of the mirror. When an array of such retroreflectors is desired, such positioning is even more difficult.
Thus, the prior art retroreflectors cannot readily be used in high precision applications such as coherent light image processing.
The prior art describes a graded refractive index rod or lens element, known by the acronym GRIN, in which an optical beam follows a substantially sinusoidal path during axial propagation. GRIN rods are described for use as image relays in "Analysis Of Refractive Index Distributions In Cylindrical, GradedIndex Glass Rods (GRIN rods) Used As Image Relays", Rawson et al, Applied Optics Volume 9, page 753 (1970). Tomlinson, III et al disclose the use of such a GRIN rod as an optical switch in U.S. Pat. No. 4,208,094. More specifically, an optical source and a plurality of optical receptors are mounted at one surface of the GRIN lens. The source is located at the lens axis and the receptors are peripherally distributed thereabout. At the opposite end of the GRIN rod is provided an angled reflector, rotatable substantially about the lens axis, for reflecting and directing light received from the light source to individual ones of the light receptors along the periphery of the opposing surface.
Although useful and interesting, such a device is incapable of functioning as a retroreflector.
There is thus a need in the prior art for an easily manufactured, compact, accurate and highly reflective retroreflector which may easily be assembled into an array to provide a retroreflective surface.